When a cast product such as a member made of aluminum is manufactured by casting operation, molten metal of aluminum is supplied into a casting die. A material of SKD61 (Japanese Industrial Standard for representing an alloy tool steel), which is excellent in strength at high temperatures, is generally adopted as a material for the casting die, because the molten metal has a high temperature.
When a heat crack and/or chipping appears in the casting die, it is difficult to obtain the member made of aluminum at a predetermined dimensional accuracy. That is, the production yield of the member made of aluminum is disadvantageously lowered. The casting die, in which the heat crack and/or the chipping appears, is replaced with a new one. However, if the replacement frequency is increased, the production cost of the member made of aluminum becomes expensive, because the casting die is generally expensive.
The heat crack is caused, for example, by a rapid change in temperature when the high temperature molten metal contacts the casting die, i.e., by thermal shock. On the other hand, the chipping is caused, for example, by cutting a soft surface layer with the member made of aluminum when the member made of aluminum is taken out from the casting die after the completion of the casting operation. Therefore, it is desirable that both of the thermal shock resistance and the hardness of the casting die are high.
Therefore, a surface treatment is usually applied to the casting die. Specifically, the surface treatment includes nitriding treatment such as the salt bath method, the gas method, and the ion method; coating treatment in which a ceramic material such as TiC and TiN is coated by means of the physical vapor deposition (PVD) method or the chemical vapor deposition (CVD) method; sulphonitriding treatment in which a mixture layer of iron sulfide and iron nitride is provided; and oxidizing treatment in which iron oxide is provided. It is also suggested in Japanese Laid-Open Patent Publication Nos. 8-144039 and 10-204610 that a plurality of treatment methods such as the nitriding treatment, the carburizing treatment, and the boronizing treatment are combined.
In recent years, the improvement of the thermal shock resistance and the hardness of the casting die is tried so that the replacement frequency of the casting die is reduced in order to reduce the production cost of cast products. However, for example, when the casting die, which is applied with the plurality of treatments as suggested in Japanese Laid-Open Patent Publication Nos. 8-144039 and 10-204610, is used, the replacement frequency is reduced to some extent as compared with a case in which a casting die applied with only the nitriding treatment is used. However, the production cost is not remarkably reduced.
It is also conceived that a material of SCM (Japanese Industrial Standard for representing one of chrome molybdenum steels), which is more inexpensive, is used as an alternative material to constitute a casting die, because the material of SKD is generally expensive. However, even when a variety of surface treatments as described above are applied to a casting die of the SCM material, it is impossible to sufficiently improve the thermal shock resistance and the hardness. Consequently, obtained casting dies do not have necessary service lives in many cases.